Methods and systems for modulation and demodulation of optical signals

ABSTRACT

An electronic device includes one or more light sources for emitting light toward a body part of a user and one or more optical sensors for capturing light samples while each light source is turned on and for capturing dark samples while the light source(s) are turned off. A signal produced by the one or more optical sensors is filtered and demodulated produce multiple demodulated signals each associated with a light source. Each signal associated with the light source(s) is analyzed to estimate or determine a physiological parameter of the user.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 62/057,089, filed on Sep. 29, 2014, and entitled “Methods and Systems for Modulation and Demodulation of Optical Signals,” which is incorporated by reference as if fully disclosed herein.

TECHNICAL FIELD

The present invention relates generally to health monitoring systems, and more particularly to health monitoring devices that include one or more optical sensors.

BACKGROUND

Health monitoring devices, such as fitness and wellness devices, may be capable of non-invasively measuring a variety of physiological characteristics of a subject via optical sensing. Such health monitoring devices can include a light source and an optical sensor.

The light source can illuminate a portion of a measurement site or even emit light that penetrates beneath the measurement site, such as into the stratum corneum of the skin or into blood vessels beneath the skin. Light from the light source may be scattered, absorbed, and/or reflected throughout the measurement site or the material forming the measurement site, such as the skin. The amount of scatter, absorption, or reflection can depend directly or indirectly on one or more physiological characteristics of the measurement site.

The optical sensor can collect light exiting the measurement site and generate electrical signals corresponding to the collected light, which may be conveyed (in the form of electrical signals) as information or data to the health monitoring device. The health monitoring device can use the optical sensor data to extrapolate, determine, derive, estimate, or measure physiological parameters of the measurement site.

In many cases, the optical sensor data may include noise associated with ambient light, surface conditions of the measurement site (e.g., cleanliness, hair, perspiration, etc.), proximity of the optical sensor and/or light source to the measurement site, and motion artifacts caused by the relative motion between the health monitoring device and the measurement site.

Furthermore, health monitoring devices often have a small form factor and are wearable by a subject for extended periods of time. The constrained proportions of such devices can limit the maximum physical size of the optical sensor and/or light source, effectively restricting the performance of both. For example, smaller light sources may emit less light and smaller optical sensors may detect less light. In addition, as the size of the optical sensor and/or light source decrease, the effects of noise increase. As a result, the accuracy, precision, and/or reliability of the physiological parameters derived from the optical sensor data can decrease with the size of many current health monitoring devices.

Accordingly, there may be a present need for an improved optical sensing system configured for use with a small form factor health monitoring device.

SUMMARY

Embodiments described herein may relate to, include, or take the form of an electronic device adapted to measures the optical characteristics, such as reflection or transmission, of a subject.

Embodiments described herein may relate to, include, or take the form of an electronic device including at least a housing with a surface adapted to be positioned proximate a measurement site of a subject, a biometric sensor positioned at least partially within the surface and including at least a plurality of light sources for emitting light toward the measurement site and an optical sensor for obtaining light exiting the measurement site. The electronic device can also include an input amplifier coupled to the output of the biometric sensor, a high pass filter coupled to the output of the input amplifier, an output amplifier coupled to the output of the high pass filter, and an analog to digital converter coupled to the output of the output amplifier.

Further embodiments described herein may relate to, include, or take the form of a sensor system including at least a plurality of light sources for emitting light toward a measurement site of a subject, an optical sensor for obtaining light exiting the measurement site, and an input amplifier coupled to the output of the biometric sensor, a high pass filter coupled to the output of the input amplifier, an output amplifier coupled to the output of the high pass filter, and an analog to digital converter coupled to the output of the output amplifier.

Additional embodiments described herein may relate to, include, or take the form of a method of optical sensing, including at least the operations of emitting light toward a measurement site of a subject, obtaining light exiting the measurement site, converting the obtained light to an electrical signal, amplifying the electrical signal to obtain an amplified signal, filtering low frequency elements from the amplified signal to obtain a filtered signal, and amplifying the filtered signal to obtain an output signal.

Other embodiments may include further including at least sampling the output signal to obtain a matrix of samples, and determining product of the matrix of samples with a demodulation matrix to obtain a demodulated matrix. Sampling the output signal to obtain a matrix of samples can include taking a first sample, taking a second sample, taking a third sample, subtracting the average of the first and third sample from the second sample to obtain an output sample, and inserting the output sample into the matrix of samples.

Other embodiments may include a configuration in which light may be emitted toward the measurement site from a plurality of light sources each with a light emitting diode, and light may be obtained exiting by an optical sensor with one of the group consisting of photodiodes, phototransistors, or optical image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to representative embodiments illustrated in the accompanying figures. It should be understood that the following descriptions are not intended to limit the disclosure to one preferred embodiment. To the contrary, each is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the described embodiments as defined by the appended claims.

FIG. 1A depicts a top plan view of an example health monitoring device.

FIG. 1B depicts a bottom plan view of the health monitoring device of FIG. 1A, showing apertures associated with an optical sensing system.

FIG. 2A depicts a detailed cross-section of FIG. 1B taken along line 2-2, showing a simplified view of the optical sensing system of FIG. 1B.

FIG. 2B depicts the detailed cross-section of FIG. 2A, showing a set of example light paths from a light source through a measurement site of a subject to an optical sensor of the optical sensing system.

FIG. 3 depicts a simplified signal flow diagram of an optical sensing system.

FIG. 4 depicts a simplified signal flow diagram of amplification and filtering stages of an optical sensing system.

FIG. 5 depicts a simplified signal flow diagram of a demodulation stage of an optical sensing system.

FIG. 6 depicts example operations of a method of calibrating an optical sensing system performed by a light source controller.

FIG. 7 depicts example operations of a method of calibrating an optical sensing system performed by an optical sensor controller.

The use of the same or similar reference numerals in different drawings indicates similar, related, or identical items where appropriate.

DETAILED DESCRIPTION

Embodiments described herein relate to systems and methods for increasing the signal to noise ratio of an optical sensing system configured for use with a small form factor health monitoring device, although the various systems and methods described herein are not limited to particular form factors and can apply equally to larger embodiments. Further, it should be appreciated that the various embodiments described herein, as well as functionality, operation, components, and capabilities thereof may be combined with other elements as necessary, and so any physical, functional, or operational discussion of any element or feature is not intended to be limited solely to a particular embodiment to the exclusion of others.

Any suitable type of electronic device can include health monitoring functionality. Example electronic devices include, but are not limited to, a smart telephone, a headset, a pulse oximeter, a digital media player, a tablet computing device, a timekeeping device, a peripheral input device (e.g., keyboard, mouse, trackpad), and a wearable device. Electronic devices that include health monitoring functionality are generally referred to herein as “health monitoring devices” or “health devices” and the like. Accordingly, it is understood that a health monitoring device as described herein is not necessarily limited to devices configured to only provide health-related information, rather, a health monitoring device may include other functionality as well.

In one embodiment, one or more sensors may be included within the health monitoring device. Sensors utilized by a health monitoring device can vary from embodiment to embodiment. Suitable sensors can include temperature sensors, electrodermal sensors, blood pressure sensors, heart rate sensors, respiration rate sensors, oxygen saturation sensors, plethysmographic sensors, activity sensors, pedometers, blood glucose sensors, body weight sensors, body fat sensors, blood alcohol sensors, dietary sensors, and so on.

Certain sensors can collect certain health-related information non-invasively. For example, a health monitoring device can include a sensor that is configured to measure changes in light absorption of a measurement site of the subject. Such a sensor can be implemented as an active sensing system including a light emitter and a light detector. In one embodiment the sensing system can include a light source for emitting light into a measurement site of a subject and an optical sensor to detect light exiting the measurement site. Light from the light source may be scattered, absorbed, and/or reflected throughout the measurement sight as a function of various physiological parameters or characteristics of the subject. For example, a subject may be a human user and the measurement site may be the user's wrist. In such an example, the tissue of the user's wrist can scatter, absorb, or reflect light emitted by the light source differently depending on various physiological characteristics of the surface and subsurface of the user's wrist. In other examples, a subject can be an animal.

For example, some embodiments may be configured to detect various subsurface events, such as the user's cardiac cycle. More particularly, during each complete heartbeat, a user's subcutaneous tissue can distend and contract, alternatingly increasing and decreasing the light absorption capacity of the measurement site. In these embodiments, the optical sensor can collect light exiting the measurement site and generate electrical signals corresponding to the collected light. Thereafter, the electrical signals can be conveyed as data to the health monitoring device. One may appreciate that such a sensor is conventionally referred to as a photoplethysmographic sensor (hereinafter “PPG sensor”).

As noted above, the performance optical sensors such as PPG sensors can be negatively affected by noise associated with ambient light, surface conditions of the measurement site (e.g., cleanliness, hair, perspiration, etc.), proximity of the optical sensor and/or light source to the measurement site, and motion artifacts caused by the relative motion between the health monitoring device and the measurement site. In other words, “environmental noise” can have a substantial impact on the quality of the data obtained by the optical sensor.

Accordingly, many embodiments described herein modulate the light output from the light source and demodulate the light collected by the optical sensors. In this manner, environmental noise can be at least partially prevented from interfering with the operation of the optical sensor. In many embodiments, modulation and demodulation can be implemented by toggling the light source on and off at a particular frequency. Thereafter, the optical sensor can generate electrical signals corresponding to the light collected from the measurement site. In these embodiments, the electrical signals can be demodulated and processed by the health monitoring device.

In many embodiments, the health monitoring device can implement a time-domain noise attenuation operation such as dark-channel subtraction. For example, in some embodiments, a sample can be taken from the optical sensor when it is known that the light source is not emitting light. This sample can be saved as a “dark” sample. Thereafter, a sample can be taken from the optical sensor when it is known that the light source is emitting light. Next, the dark sample can be subtracted from this “light” sample to remove the effects of noise from the light sample. In other words, a dark sample can represent an approximation of the amount of environmental noise at or near the time the dark sample was taken.

In further embodiments, a second dark sample can be taken after the light sample. In such cases, the first and second dark samples can be averaged. The averaged dark sample may be subtracted from the intervening light sample in order to remove the effects of noise from the light sample.

Although time-domain noise attenuation operations such as dark-channel subtraction may be suitable in certain embodiments for mitigating the effects of certain environmental noise sources, the delays required between “light” samples (e.g., delays where dark samples are required to be taken) can cause undesirable aliasing. In some examples, using dark-channel subtraction may require a reduction in sampling rate, which in turn can increase the number of aliases present in the data output from the optical sensor. In these examples, low frequency aliases may be particularly undesirable.

For example, many physiological signals measurable by a PPG sensor (e.g., respiration, heart rate) are relatively low frequency signals. For example, a healthy user's heart rate may vary from less than one beat per second to only a few beats per second. In other words, a heart rate may be a physiological signal within the frequency band from 0 Hz to 3-4 Hz. Accordingly, low frequency aliasing (as a result of dark-channel subtraction) can interfere with the detection and extraction of low frequency physiological signals, such as heart rate.

Accordingly, certain embodiments described herein implement a filter prior to time-domain noise attenuation. For example, a high pass or a band pass filter can be implemented to attenuate some or all frequencies except frequencies nearby the modulation frequency. In this manner, certain frequencies of environmental noise can be attenuated prior to time-domain noise attenuation operations such as dark-channel subtraction. As a result, low frequency noise sources are attenuated and, accordingly, the negative effects of low frequency aliasing can be reduced.

Although filtering prior to dark-channel subtraction may be suitable in some embodiments for attenuating certain noise sources, the process of filtering with a dynamic filter (such as a band pass or high pass filter) can increase the complexity of demodulating and/or demultiplexing the signal. For example, the time response of a dynamic filter can mix the time-multiplexed signals sent from the light source in time. In other words, dynamic filtering of a signal S₀ received at time t₀ can cause the signal S₀ to interfere with a signal S₁ sent at time t₁.

Accordingly, embodiments described herein relate to methods and systems for demodulating and demultiplexing signals output from an optical sensor implementing both dynamic filtering and time-domain noise attenuation.

FIG. 1A depicts a top plan view of an example health monitoring device 100. In the illustrated embodiment, the health monitoring device may be implemented as a portable electronic device that is adapted to be worn by a user. Other embodiments can implement the health monitoring device differently. For example, the health monitoring device can be a smart phone, a gaming device, a digital music player, a sports accessory device, a medical device, a device that provides time and/or weather information, a health assistant, and other types of electronic device suitable for attaching to a user.

The health monitoring device can be implemented as a wearable health assistant that provides health-related information (whether real-time or not) to the user, authorized third parties, and/or an associated monitoring device. The wearable health assistant may be configured to provide health-related information or data such as, but not limited to, heart rate data, blood pressure data, temperature data, blood oxygen saturation level data, diet/nutrition information, medical reminders, health-related tips or information, or other health-related data. The associated monitoring device may be, for example, a tablet computing device, phone, personal digital assistant, computer, and so on.

As another example, the health monitoring device can be configured in the form of a wearable communications device. The wearable communications device may include a processor coupled with or in communication with a memory, one or more sensors, one or more communication interfaces, output devices such as displays and speakers, one or more input devices, and a health monitoring system. The communication interface(s) can provide electronic communications between the communications device and any external communication network, device or platform, such as but not limited to wireless interfaces, Bluetooth interfaces, USB interfaces, Wi-Fi interfaces, TCP/IP interfaces, network communications interfaces, or any conventional communication interfaces. The wearable communications device may provide information regarding time, health, statuses or externally connected or communicating devices and/or software executing on such devices, messages, video, operating commands, and so forth (and may receive any of the foregoing from an external device), in addition to communications.

The health monitoring device 100 includes a housing 102 at least partially surrounding a display 104. In many examples, the display 104 may incorporate an input device configured to receive touch input, force input, and the like and/or output information to a user, such as various health parameters or health-related suggestions. The health monitoring device 100 may also include one or more buttons or input devices (not shown). The housing 102 can form an outer surface or partial outer surface and protective case for the internal components of the health monitoring device 100. In the illustrated embodiment, the housing 102 is formed into a substantially rectangular shape, although this configuration is not required.

The housing 102 can be formed of one or more components operably connected together, such as a front piece and a back piece or a top clamshell and a bottom clamshell. Alternatively, the housing 102 can be formed of a single piece (e.g., uniform body or unibody) operably connected to the display 104.

The display 104 can be implemented with any suitable technology, including, but not limited to, a multi-touch sensing touchscreen that uses liquid crystal display (LCD) technology, light emitting diode (LED) technology, organic light-emitting display (OLED) technology, organic electroluminescence (OEL) technology, or another type of display technology. A button (not shown) might take the form of a home button, which may be a mechanical button, a soft button (e.g., a button that does not physically move but still accepts inputs), an icon or image on the display 104 or on an input region, and so on. Other buttons or mechanisms can be used as input/output devices, such as a speaker, rotary input, a microphone, an on/off button, a mute button, or a sleep button.

The health monitoring device 100 may include one or more health sensors. In some examples, the health sensors can be disposed on a bottom surface 112 of the housing 102. For example, FIG. 1B depicts a bottom plan view of the health monitoring device of FIG. 1A, showing two apertures 114 a, 114 b associated with an optical sensing system that can be used to obtain health-related information from a user. As noted above, an optical sensing system can include a light source and an optical sensor (not shown). The light source can be disposed within a first aperture 114 a and the optical sensor can be disposed within a second aperture 114 b. In some embodiments, the light source may fill the first aperture and/or the optical sensor may fill the second aperture, while in other embodiments optically-transparent windows or covers may seal the light source and optical sensor in their respective apertures. As used herein, the term “optically transparent” and variants thereof does not necessarily mean that the structure is transparent to visible light but instead to the particular wavelength of light emitted by the light source and/or received by the sensor. Thus, some windows may pass infrared light but block visible light and still be optically transparent.

In some examples, and as illustrated, the apertures 114 a, 114 b the bottom surface 112 of the housing 102 may take a convex shape, although this configuration is not required. In these examples, the apertures 114 a, 114 b can be formed symmetrically about the apex of the convex curvature of the bottom surface 112. In this manner, the curvature of the housing itself can provide an optical barrier between the light source and the optical sensor. For example, the apex of the convex curvature of the external surface can physically and optically separate the light source from the optical sensor. In other examples, the bottom surface 112 can be flat, faceted, or concave. In further embodiments, the bottom surface 112 can take an arbitrary shape. In some embodiments, the light source and/or optical sensor may be recessed within the apertures such that there is no direct light path between the two.

The apertures 114 a, 114 b may be separated by a selected distance. The distance between the apertures 114 a, 114 b can vary from embodiment to embodiment.

FIG. 2A depicts a detailed cross-section of FIG. 1B taken along line 2-2, showing a simplified view of the optical sensing system of FIG. 1B. The optical sensing system includes a light source 116 and an optical sensor 122. In one embodiment the light source 116 may be light emitting diode configured to emit light at a particular frequency. For example, in certain embodiments the light source 116 can be configured to emit green light. In another example, the light source 116 can emit infrared light. In still further examples, the light source 116 can be configured to emit light in another frequency band.

In many embodiments, more than one independent light source can be implemented as the light source 116. For example, certain embodiments can include four light sources within the space reserved for the light source 116. In other embodiments more or fewer light sources can be used. In these and related embodiments, the multiple individual light sources 116 can be configured to illuminate independently or at once. For example, in certain embodiments, individual light sources 116 can be illuminated in a sequence.

The optical sensor 122 can be a photodiode, phototransistor, and/or an optical image sensor such as a charge-coupled device (“CCD”) or complementary metal-oxide semiconductor (“CMOS”) array.

The light source 116 can be disposed within a cavity 118 that extends to the aperture 114 a. In some embodiments, the cavity 118 may have a shorter width than that of the aperture 114 a, although this configuration is not required.

A lens 120 or window may seal the cavity 118; the lens 120 need not focus or scatter light in any particular fashion. The lens 120 may be formed from the same material as the bottom surface 112, although this is not required. In one embodiment, the lens 120 can be configured to diffuse light, while in other embodiments it may focus light or may not affect the directionality of light passing therethrough. In some embodiments, the lens 120 may be optically transparent. In still further embodiments, the lens 120 may be configured to exhibit transparency in a first light frequency band and to be opaque in a second light frequency band. For one example, the first light frequency band can be infrared light and the second light frequency band can be visible light.

As with the light source 116, the optical sensor 122 can be disposed within a cavity 124 and below another window or lens 126. As with the lens 120, the lens 126 can be configured to diffuse light. In other embodiments, the lens 126 may be optically transparent, or may be shaped to focus light to a particular focal point (or not). In still further embodiments, the lens 126 may be configured to exhibit transparency in a first light frequency band and to be opaque in a second light frequency band. For one example, the first light frequency band can be infrared light and the second light frequency band can be visible light.

FIG. 2B depicts a second cross-section like that of FIG. 2A, now showing a set of example light paths from a light source through a measurement site 110 of a subject to an optical sensor of the optical sensing system. As illustrated, light emitted from the light source 116 passes into an illumination area 130 of the measurement site 110, through an intermediate volume 128 of the measurement site 110, to a collection area 132 of the measurement site 110, and, thereafter, exits the collection area 132 where it may be collected by the optical sensor 122 and conveyed to the health monitoring device 102. In many embodiments, the subject may be a human user.

As illustrated, a portion of the light emitted from the light source 116 may not exit the intermediate volume 128 through the collection area 132. For example, some light may continue transmitting in a direction parallel to the length of the intermediate volume 128. In addition, some light can transmit in directions away from the collection area 132. In addition, some light can be absorbed within the intermediate volume collection area 132, depicted in FIG. 2B as point-terminated lines. Accordingly, the quantity of light received by the optical sensor 122 may be less than the quantity of light emitted by the light source 116.

FIG. 3 depicts a simplified signal flow diagram of an optical sensing system. An optical sensing system can include a light source controller 300 that is coupled to one or more light sources 302. The light source controlled 300 can be implemented as a microprocessor, integrated circuit, with discrete circuitry, or using any other suitable technique. The light source controller 300 can be configured to control the power state, brightness, and/or color of the light emitted by the one or more light sources 302. For example, in some embodiments the light source(s) 302 can include a light configured to emit green light and a second light configured to emit infrared light. In such an embodiment, the light source controller 300 can alternately illuminate the green light and the infrared light. In another example, the light source controller 300 can illuminate the green light and the infrared light at the same time.

The optical sensing system of FIG. 3 can also include an optical sensor controller 306 that is configured to receive output from the one or more optical sensors 308. The optical sensors 308 can receive light from the light source 302 that has reflected or transmitted through a measurement site 310 of a subject. In many embodiments, the subject may be a human user.

The optical sensor controller 306 can be implemented as a microprocessor, integrated circuit, with discrete circuitry, or using any other suitable technique. The optical sensor controller 306 can also be coupled to the light source controller 300. In many examples, the optical sensor controller 306 can use information provided by the light source controller 300. For example, in one embodiment, the light source controller 300 can inform the optical sensor controller 306 that the light source controller 300 is not illuminating any of the light sources 302. Alternatively, the light source controller 300 can inform the optical sensor controller 306 that the light source controller 300 is illuminating a particular light source 302.

In some embodiments described herein, the optical sensor controller 306 can implement one or more operations in an attempt to attenuate environmental noise. In one embodiment, the optical sensor controller 306 can implement a dark-channel subtraction operation. As described above, a dark-channel subtraction operation can take a sample output from an optical sensor 308 when the light source is not illuminated in order to determine the amount of ambient light received by the sensor when the light sources are “dark.” Shortly thereafter, a light source 302 can be illuminated and another sample can be taken. Next, the “dark” sample can be subtracted from the “light” sample.

The optical sensor controller 306 can implement the dark-channel subtraction operation with assistance from the light source controller 300. For example, the light source controller 300 can inform the optical sensor controller 306 when light sources are on or off. In alternate embodiments, the optical sensor controller 306 can direct the light source controller 300 to turn particular light sources 302 on or off.

In further embodiments, the light source controller 300 and/or the optical sensor controller 306 can be implemented as software instructions to be executed by a processor associated with a health monitoring device.

FIG. 4 depicts a simplified signal flow diagram of amplification and filtering stages of an optical sensing system. In the illustrated embodiment, the optical sensor 400 can receive light. In many examples, the light may be transmitted from a light source associated with the optical sensing system, but, as noted above, additional environmental light can be received by the optical sensor as well.

After the light is received by the optical sensor 400, the optical sensor 400 may convert the light into analog electrical signals. In many examples, the optical sensor 400 can be a photodiode. Next, the analog electrical signal can be received by a transimpedance amplifier at 402. In many examples, a transimpedance amplifier can be suitable to convert the current output from a photodiode to a usable voltage. In such examples, measuring the current changes through a photodiode may be preferable to measuring the voltage changes across a photodiode because current may vary more linearly with changes in light than voltage. In one embodiment, the photodiode can be zero biased. In other examples the photodiode can be reverse biased.

After the analog electrical signal is amplified by the transimpedance amplifier 402, the amplified analog electrical signal can be passed through a high pass filter at 404. As noted with respect to embodiments described above, a high pass filter can be useful to remove low frequency components within the electrical signal that might otherwise interfere with detection of low frequency physiological parameters such as heart rate.

As noted above, the cutoff frequency of the high pass filter 404 can be selected to be below the frequency of modulation of light expected from the light source. For example, if the light source is modulating at 1 KHz, the cutoff frequency of the high pass filter can be 750 Hz. In other embodiments, the cutoff frequency can be closer or father away from the modulation frequency.

In other examples, the filter can be implemented as a band pass filter. For example, if the light source is modulating light at 1 KHz, a band pass filter can have a lower cutoff at 750 z and a high cutoff at 1.25 KHz. In other examples, different cutoff frequencies can be used.

In many examples, the filter can also remove direct current biases present within the signal.

After the signal is filtered by the high pass filter 404, it can be passed to a programmable gain amplifier at 406 and thereafter to an analog to digital converter at 408. The programmable gain amplifier 406 can increase the dynamic range of the optical sensor. For example, certain users may have darker skin than other users. Accordingly, the amount of light absorbed by the measurement site can vary from person to person. In these embodiments, the programmable gain amplifier 406 can be adjusted on a per-user basis in order to provide the maximum signal to the analog to digital converter 408.

In other examples, the gain of the programmable gain amplifier 406 can be based, at least in part, on feedback from the analog to digital converter 408. For example, if the analog to digital converter 408 determines that the average signal output from the programmable gain amplifier 406 is too low, then the analog to digital converter 408 can cause the programmable gain amplifier 406 to increase gain by a certain amount. In other examples, if the analog to digital converter 408 determines that the average signal output from the programmable gain amplifier 406 is too high (e.g., clipping is detected), then the analog to digital converter 408 can cause the programmable gain amplifier 406 to decrease gain by a certain amount.

FIG. 5 depicts a simplified signal flow diagram of a demodulation stage of an optical sensing system. In the illustrated embodiment, a digital signal can be received and sampled into a matrix at 502. Thereafter at 504, the matrix can be multiplied by a demodulation matrix in order to obtain a substantially noise-free measurement of the physiological parameters of a measurement site.

A demodulation matrix can be calculated in a number of ways. For example, in certain embodiments, a demodulation matrix can be fixed. In other embodiments, a different demodulation matrix can be selected depending upon particular environmental conditions. In still further embodiments, a plurality of demodulation matrices can be iterated through in order to determine which demodulation matrix obtains the highest quality output signal.

In still further embodiments, a demodulation matrix can be calculated dynamically and/or during a calibration procedure. For example, as with other embodiments described herein, an optical sensing system can have a plurality of light sources, a measurement site, and an optical sensor.

In many embodiments, the light sources can output modulated light. For example, if an embodiment is implemented with four light sources, a single light source can be illuminated at a time. Once all light sources have been illuminated, the cycle can repeat. In other words, the output of light from a light sensor may be periodic with period T, sampled for k periods. The function can be modeled as: x(t)=x(t+kT)  Equation 1

In one example, the function x(t) may be the light as it reaches the measurement site. In other words, light that has been affected by physiological parameters of the measurement site, but that is not yet accompanied or affected by environmental noise. As noted above, the light may pass through the measurement site and may be joined by other noise sources. In other words, the light can pass through a linear time invariant system having an unknown impulse. Such a function can be modeled as H(t). Accordingly, once the light reaches the optical sensor, it may have been affected by the linear time invariant system. In other words, the light that reaches the sensor y(t) can be modeled as the convolution of x(t)*H(t): y(t)=∫_(−∞) ^(∞) H(τ)x(t−τ)dτ=y(t+kT)  Equation 2

However, as noted above, in some embodiments, multiple light sources can be used. Accordingly, the original signal x(t) can be represented by a sum of n individual signals, each having a unique scaling factor α associated with it: x(t)=Σ_(i=1) ^(n)α_(i)(t)x _(i)(t)  Equation 3

Because light noise may not be modulated at the modulated frequency, the scaling factors may α vary and/or correspond to the physiological parameters of the measurement site.

In many embodiments, it can follow from Equation 3 that the scaling factors can pass through the linear time invariant system H(t), to be directly measured when measuring y(t): y(t)=Σ_(i=1) ^(n)α_(i)(t)y _(i)(t)  Equation 4

In other words, the measured signal y(t), may be composed of a number of individual signals y_(i)(t); once the individual signals y_(i)(t) can be demultiplexed and demodulated, the individual scaling factors, and thus the physiological parameters of the measurement site, can be determined.

In certain embodiments, various assumptions can assist in demuxing and demodulating the signal y(t). For example, by turning on a single light source and/or turning of all light sources, the signal x(t) can be computationally simplified. Also, in embodiments directed to detecting low frequency physiological signals such as heart rate, it can be assumed that the scaling factors do not change between samples of particularly high sampling rates.

Also as noted above, dark-channel subtraction may be implemented while demodulating to attempt to remove as much noise as possible before further signal processing. For example, a demodulated signal z_(j)(k) can be formed from synchronous samples of y(t) at individual time instances τ_(j). In sync with sampling, the light source can turn on and off at known rate. In this manner, time instances τ_(j) can fall immediately between dark periods of a light source. In other words, a dark sample can be taken at τ_(j)−Δ_(j) and another dark sample can be taken at τ_(j)+Δ_(j). Between the dark samples, at τ_(j), a “light” sample can be taken. The dark channel subtraction can be modeled as:

$\begin{matrix} {{z_{j}(k)} = {{y\left( {t_{0} + {k\; T} + \tau_{j}} \right)} - \frac{{y\left( {t_{0} + {kT} + \tau_{j} - \Delta_{j}} \right)} + {y\left( {t_{o} + {kT} + \tau_{j} + \Delta_{j}} \right)}}{2}}} & {{Equation}\mspace{14mu} 5} \end{matrix}$

In many embodiments, the demodulated signal z_(j)(k) can include m samples. In other words, Equation 5 can be defined from j=1 . . . m.

As noted above, individual light sources can be turned on or off. Accordingly, in some embodiments, one light source i may be turned on a time to simplify the calculation of demodulation. More specifically, a demodulated signal z_(j)(k) that relates specifically to one light source i can be modeled as: z _(j,i)(k)=α_(i)(t)y _(i)(t)  Equation 6

Which in turn can be modeled as: z _(j,i)(k)=α_(i)(t ₀ +kT+τ _(j))ω_(j,i)  Equation 7

Where:

$\begin{matrix} {\omega_{j,i} = {{y\left( {t_{0} + {kT} + \tau_{j}} \right)} - \frac{{y\left( {t_{0} + {kT} + \tau_{j} - \Delta_{j}} \right)} + {y\left( {t_{0} + {k\; T} + \tau_{j} + \Delta_{j}} \right)}}{2}}} & {{Equation}\mspace{14mu} 8} \end{matrix}$

In order to simplify the calculation, some embodiments can utilize the presumption that because the Equations 1-8 are time invariant, then t₀=0. In some embodiments, the simplified calculation can be modeled as: z _(j,i)(k)=α_(i)(kT+τ _(j))ω_(j,i)  Equation 9

Where:

$\begin{matrix} {\omega_{j,i} = {{y\left( \tau_{j} \right)} - \frac{{y\left( {\tau_{j} - \Delta_{j}} \right)} + {y\left( {\tau_{j} + \Delta_{j}} \right)}}{2}}} & {{Equation}\mspace{14mu} 10} \end{matrix}$

Accordingly, in certain embodiments, the dark-channel subtraction demodulation operation can be accomplished by calculating ω_(j,i). In other words, because z_(j,i)(k) is measureable, the only unknowns in Equation 9 are, generally speaking, ω_(j,i) and α_(i)(kT+τ_(j)). However, after the demodulation process, one may appreciate that demultiplexing may still be required.

Accordingly, certain embodiments may attempt to estimate ω_(j,i). For example, all individual light sources i=1 . . . n can be demultiplexed as described above with respect to the examples of Equations 5-10. In such embodiments, an individual light source can be illuminated while all other light sources are turned off, and the light source can be sampled a certain number of times. As described above, each “sample” can actually constitute three samples; two “dark” samples can be averaged and subtracted from a single “light” sample. Once m samples are taken of the selected light source, the next light source can be selected.

For example, lights sources i=1 . . . n can be sampled with samples j=1 . . . m, and thus all measured results can be modeled as a matrix:

$\begin{matrix} {{Z(k)} = \begin{bmatrix} z_{1,1} & \ldots & z_{1,m} \\ \vdots & \ddots & \vdots \\ z_{n,1} & \ldots & z_{n,m} \end{bmatrix}} & {{Equation}\mspace{14mu} 11} \end{matrix}$

Correspondingly, the demodulation values can be modeled as a matrix:

$\begin{matrix} {W = \begin{bmatrix} \omega_{1,1} & \ldots & \omega_{1,m} \\ \vdots & \ddots & \vdots \\ \omega_{n,1} & \ldots & \omega_{n,m} \end{bmatrix}} & {{Equation}\mspace{14mu} 12} \end{matrix}$

One may note that the demodulation values does not vary with k. In addition, the scalar coefficients can be modeled as a matrix:

$\begin{matrix} {{A(k)} = \begin{bmatrix} \alpha_{1{({kT})}} & \ldots & 0 \\ \vdots & \ddots & \vdots \\ 0 & \ldots & \alpha_{n{({kT})}} \end{bmatrix}} & {{Equation}\mspace{14mu} 13} \end{matrix}$

Alternatively, some embodiments can model the system as the matrix equation, presuming that W is full rank and that n=m: Z(k)=A(k)W  Equation 14

Thus, in order to calculate the scaling factors A(k), given only the measured values from the optical sensor, Z(k), some embodiments utilize the simplification: A(k)=Z(k)W ⁻¹  Equation 15

In this configuration, the matrix W⁻¹ represents demultiplexing constants.

However, in many embodiments, Equation 15 is only useful once and unless the demultiplexing constants are known and/or estimated. Accordingly, in some embodiments, an initial matrix of A_(init) can be supplied. For example, an identity matrix may be used in certain embodiments:

$\begin{matrix} {A_{init} = \begin{bmatrix} 1 & \ldots & 0 \\ \vdots & \ddots & \vdots \\ 0 & \ldots & 1 \end{bmatrix}} & {{Equation}\mspace{14mu} 16} \end{matrix}$

In other embodiments, an initial matrix of A_(init) can be supplied from the diagonal of the measured matrix Z(k). For example, an identity matrix may be used in certain embodiments:

$\begin{matrix} {A_{init} = \begin{bmatrix} z_{1,1} & \ldots & 0 \\ \vdots & \ddots & \vdots \\ 0 & \ldots & z_{n,m} \end{bmatrix}} & {{Equation}\mspace{14mu} 17} \end{matrix}$

Thus, in some embodiments, by multiplying the matrix inverse of the measured values Z(k)⁻¹ with the initial matrix A_(init), a first estimation of the demultiplexing constants W⁻¹ can be calculated. For example: A _(init) Z(k)⁻¹ =W ⁻¹  Equation 18

In some embodiments, the demultiplexing constants can be calculated and averaged with previous demultiplexing constants. After a sufficient number of calculations of the demultiplexing constants, returning to FIG. 5, a sampled signal matrix at 502 can be multiplied by a demultiplexing matrix at 504 to obtain scalar constants corresponding to physiological parameters of the measurement area.

FIG. 6 depicts example operations of a method of calibrating an optical sensing system performed by a light source controller. The method can begin at operation 600 in which a light source controller receives a calibration signal. Next at 602, all lights controlled by the light source controller can be turned off for a selected period of time. In many examples, operation 602 may be a first “dark” sample collection period of a dark-channel subtraction process. Next at 604, one individual light can be turned on for a selected period of time. In many examples, operation 605 may be the “light” sample of a dark-channel subtraction process. Next at 606, all lights controlled by the light source controller can be turned off for another selected period of time. In many embodiments, operation 606 can be a second “dark” sample collection period of a dark-channel subtraction process. Lastly at operation 608, the method can cycle back to operation 602 after selecting the next light source to illuminate individually.

The order in which lights are illuminated can vary from embodiment to embodiment. For example, certain embodiments may illuminate adjacent lights sequentially. In other examples, a random or pseudorandom illumination pattern can be used.

FIG. 7 depicts example operations of a method of calibrating an optical sensing system performed by an optical sensor controller. The method can begin at operation 700 in which a calibration signal is sent by an optical sensor controller to a light source controller. Next at operation 702, a demultiplexing matrix can be determined and/or estimated. For example, as described with respect to FIG. 5. The method can continue to operation 704 at which a number of calculated demultiplexing matrices are calculated and averaged. The method can conclude at operation 706 during which the average demultiplexing matrix is saved for future use.

For example, a saved demultiplexing matrix can be used in conjunction with Equation 15, modified to account for all light sources being active: [α₁(kT) . . . α_(n)(kT)]=[z ₁(kT) . . . z _(n)(kT)]W ⁻¹  Equation 19

Many embodiments of the foregoing disclosure may include or may be described in relation to various methods of operation, use, manufacture, and so on. Notably, the operations of methods presented herein are meant only to be exemplary and, accordingly, are not necessarily exhaustive. For example an alternate operation order, or fewer or additional steps may be required or desired for particular embodiments.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not meant to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. Likewise, the features of the different embodiments may be exchanged, substituted, or omitted where compatible and appropriate. 

We claim:
 1. An electronic device comprising: a housing comprising a surface adapted to be positioned proximate a measurement site of a subject; a biometric sensor positioned at least partially within the surface and comprising: a plurality of light sources for emitting light toward the measurement site at a selected modulation frequency; and an optical sensor for obtaining light exiting the measurement site; and an input amplifier coupled to the output of the biometric sensor and disposed within the housing; a high pass filter coupled to an output of the input amplifier and disposed within the housing, the high pass filter having a cutoff frequency above that of a periodic biometric property of the measurement site; an output amplifier coupled to an output of the high pass filter and disposed within the housing; and an analog to digital converter coupled to an output of the output amplifier and disposed within the housing.
 2. The electronic device of claim 1, wherein: each light source of the plurality of light sources comprises a light emitting diode; and the optical sensor comprises one of the group consisting of photodiodes, phototransistors, or optical image sensors.
 3. The electronic device of claim 1, wherein the input amplifier comprises a transimpedance amplifier.
 4. The electronic device of claim 1, wherein the output amplifier comprises a programmable gain amplifier.
 5. The electronic device of claim 1, further comprising a processor coupled to the output of the analog to digital converter.
 6. The electronic device of claim 1, wherein the processor is configured to control each of the plurality of light sources of the biometric sensor.
 7. The electronic device of claim 6, wherein the biometric sensor further comprises a measurement mode and a calibration mode.
 8. The electronic device of claim 7, wherein the calibration mode comprises: deactivating all light sources for a selected time period; activating a first light source of the plurality of light sources for a selected time period; and deactivating all light sources for a selected time period.
 9. A sensor system for measuring a periodic biometric characteristic of a subject, the sensor system comprising: a plurality of light sources operative to emit a series light pulses at a selected modulation frequency toward a measurement site of the subject; an optical sensor operative to receive light exiting a portion of the measurement site, the light originating from the series of light pulses; a transimpedance input amplifier coupled to the output of the optical sensor; a high pass filter coupled to the transimpedance input amplifier and configured to filter frequencies lower than the selected modulation frequency; an output amplifier coupled of the high pass filter and configured to amplify an output of the high pass filter; and an analog to digital converter coupled to the output amplifier configured to associate a signal amplitude of the output amplifier with a digital value.
 10. The sensor system of claim 9, wherein: each of the light source of the plurality of light sources comprises a light emitting diode configured to emit light at a selected frequency; and the optical sensor comprises one of the group consisting of photodiodes, phototransistors, and optical image sensors.
 11. The sensor system of claim 9, wherein the input amplifier comprises a transimpedance amplifier; and the output amplifier comprises a programmable gain amplifier.
 12. The sensor system of claim 9, wherein: a first subset of the plurality of light sources comprises a plurality of light emitting diodes configured to emit light at a first selected frequency; and a second subset of the plurality of light sources comprises a plurality of light emitting diodes configured to emit light at a second selected frequency; wherein when the first subset is emitting light the second subset is prevented from emitting light.
 13. The sensor system of claim 9, further comprising a processor coupled to the output of the analog to digital converter.
 14. The sensor system of claim 9, wherein the processor is configured to control the modulation of at least one of the light sources of the plurality of light sources.
 15. The sensor system of claim 14, wherein the biometric sensor further comprises a measurement mode and a calibration mode.
 16. The electronic device of claim 15, wherein the calibration mode comprises: deactivating all light sources for a selected time period; activating a first light source of the plurality of light sources for a selected time period; and deactivating all light sources for a selected time period.
 17. A method of optical sensing, comprising: emitting light toward a measurement site of a subject at a selected modulation frequency; obtaining light exiting the measurement site; converting the obtained light to an electrical signal; amplifying the electrical signal to obtain an amplified signal; filtering low frequency components below the selected modulation frequency from the amplified signal to obtain a filtered signal; and amplifying the filtered signal to obtain an output signal.
 18. The method of claim 17, further comprising: sampling the output signal to obtain a matrix of samples; and determining product of the matrix of samples with a demodulation matrix to obtain a demodulated matrix.
 19. The method of claim 18, wherein sampling the output signal to obtain a matrix of samples comprises: taking a first sample; taking a second sample; taking a third sample; subtracting the average of the first and third sample from the second sample to obtain an output sample; and adding the output sample the matrix of samples.
 20. The method of claim 17, wherein: light is emitted toward the measurement site from a plurality of light sources each comprising a light emitting diode; and light is obtained exiting by an optical sensor comprising one of the group consisting of photodiodes, phototransistors, or optical image sensors. 